In industrial and military nuclear operations, radioactive wastes ("radwastes") are formed which must be treated in a suitable manner to permit safe storage. Thus, most conventional processes whereby uranium, plutonium, and/or other radionuclides are recovered via the reprocessing of nuclear fuel rods yield radwastes as necessary, but unwanted, by-products. Because of the highly radioactive nature of those materials, they must be prevented from reaching the biosphere. The composition of any particular radwaste will, self-evidently, be dependent upon the nuclear technology involved. An analysis of a typical radwaste resulting from the reprocessing of nuclear fuel rods is provided by A. E. Ringwood et al. in "Immobilization of High Level Nuclear Reactor Wastes in SYNROC", Nature, 278, Mar. 15, 1979, pages 219-223:
______________________________________ Mol % ______________________________________ Fission Products Rare Earth 26.4 Zr 13.2 Mo 12.2 Ru 7.6 Cs 7.0 Pd 4.1 Sr 3.5 Ba 3.5 Rb 1.3 Actinides U + Th 1.4 Am + Cm + Pu + Np 0.2 Processing Contaminants Fe 6.4 (PO.sub.4) 3.2 Na 1.0 Others Tc + Rh + Te + I + Ni + Cr 9.0 ______________________________________
As is apparent, radwaste is a complex and many-component mixture.
Various schemes have been proposed to store radioactive wastes so as to isolate them from the environment. Inasmuch as the radioactivity of the subject materials may, in some instances, persist for literally thousands of years, the means of isolation must withstand attack essentially indefinitely from not only the radioactive material, but also from the chemical and physical stresses of the natural environment. Most of those proposals have contemplated first reducing the volume of the effluents, thereby concentrating the radioactive substances, and thereafter incorporating the concentrates into a surrounding matrix.
The earliest procedures simply solidified the wastes in alkali metal silicates, cements, concrete, or bitumen. Modifications of those processes have involved incorporating the radioactive concentrates into polymerizable resins and subsequently polymerizing the resins to solid blocks. Unfortunately, all of those matrices have low thermal stability and relatively low radiation resistance over extended periods of time. Moreover, those matrices exhibit unacceptable mechanical stability and leaching resistance.
Another approach has comprehended mixing the waste concentrates with various clays, diatomaceous earth, ion exchangers, peat, vermiculite, asbestos, and/or other ceramic materials and thereafter firing the mixture with or without concurrent pressing to form a solid body. However, the general quality of the solidified bodies containing high concentrations of radioactive substances has not been sufficient for final storage purposes and the physical stability and chemical durability of the products have not been deemed satisfactory for long term storage.
Probably the most extensive area of research has involved the use of glass as a matrix material. Thus, glass is capable of dissolving a wide variety of substances, including the radioactive isotopes of concern in the waste concentrates. Hence, the concentrated wastes are mixed with various glass formers, the mixture is melted so as to distribute the radioactive substances throughout the resulting glass melt, and the melt is then cooled to a glass body. Concern has been expressed, however, regarding the fundamental lack of thermodynamic stability inherent in glass. It is well-recognized that, under hydrothermal conditions, glasses are subject to leaching, dissolution, and uncontrolled crystallization which often produces surface spalling. Furthermore, decomposition of the glass structure may occur in the course of prolonged storage due to the continued emission of radiation and heat energy by the incorporated radioactive substances, with the result that the resistance of the glass structure deteriorates with time such that its ability to effectively retain radioactive materials is diminished.
Most recently, hot pressed ceramic bodies of exotic compositions have been hailed as more stable host media with superior thermal stability and corrosion resistance. Examples of such are discussed by Ringwood et al., supra. Unfortunately, those materials have proven difficult to mass produce commercially without undesirable porosity and inhomogeneity.